There is in some parts of New England a
kind of tree...whose juice that weeps out of its incision, if it be
permitted slowly to exhale away the excess moisture, doth congeal into a
sweet and saccharine substance.

IntroductionAn
event associated with spring is the production of maple syrup from the sap of
the sugar maple tree (Acer saccharum Marsh.). This process, which was
originally discovered by Native Americans, is one of the relatively few uniquely
North American crops and it is one of the oldest American crops (Willits, 1958).
The monks of St. John�s have made syrup since 1942 when, perhaps spurred by
sugar shortages during the war, the monks tapped 150 trees, collected 1440
gallons of sap, and boiled it down in the candle shop to make their first 45
gallons of pure maple syrup. This springtime tradition has continued until the
present. During today�s lab we will visit the St. John�s maple sugar operation
and learn more about this monastic ritual.

Sap ChemistryWhen a sugar maple tree is tapped, sap flows out
of the hole. A single tap will produce about 10 gallons of sap per per and
yield approximately one quart of finished syrup. The sap contains sucrose and
trace amount of oligosaccharides including raffinose (Willits, 1958). The
concentration of sucrose in the sap is typically 2-3%, though it can range from
0.5 - 10% (Kozlowski & Pallardy, 1997). Environmental conditions can affect the
yield of sugar. Trees grown with adequate moisture and fertilizer produce
higher yields than trees in infertile soil and dry conditions. Sap yield is also
lower if leaves are defoliated in the previous season. Trees with an exposed
crown produce greater amounts of sap than trees grown under crowded conditions -
presumably because of the advantage due to photosynthesis (Kozlowski & Pallardy,
1997).

Other organic compounds in the sap include organic acids, amino acids, amides,
ammonia, and peptides. The organic acids in the sap include malic (0.21%),
citric (0.002%), and traces of succinic, fumaric and several others. The total
ash (mineral) content of the sap is 0.66 %. Common minerals include potassium
(0.26%), calcium (0.07%), silicon oxide (0.02%) and lesser amounts of manganese,
sodium, and magnesium (Willets, 1958).

Syrup-MakingTo make maple syrup, the sap is concentrated by
boiling to yield a solution which must legally contain 66.7% sugar or have a
specific density of 66.5 degrees Brix or 36 degrees Baum. In practice a
syrup-maker uses a hydrometer to measure the density of the syrup. An alternate
method is to monitor temperature - the syrup is finished when it boils at 7 F
above the boiling point of water.

It takes approximately 40 gallons of sap to produce one
gallon of syrup. Thus, there is a 40 to one ratio of sap-to-syrup. The lower
this ratio the better because it means that less boiling is required to produce
a gallon of syrup. At St. John�s our average sap/syrup ratio is almost exactly
40. However, early in the season our sap/syrup ratio is usually slightly higher
(50 or 60 to one) but it declines toward the end of the season. The reason this
occurs is because the concentration of sugar in the sap varies with the season.

To estimate the number of gallons of sap needed to produce a gallon of syrup
(sap-to-syrup ratio), a syrup-maker can use the Rule of 86 (Willits, 1958) which
is mathematically expressed as follows:

This equation is
derived from the fact that finished syrup has 86.3% solids. Thus, to calculate
how much sap is required to produce one gallon of syrup, divide 86 by the sugar
concentration of the sap. For example, the sugar concentration in trees at St.
John's is typically 2.0%. Thus, according to the Rule of 86, it would take
approximately 43 gallons of sap (86/2 = 43) to make one gallon of syrup. Or, in
other words, you must boil off 42 gallons of water to leave one gallon of maple
syrup (Willits, 1958).

We can rearrange Equation 1 to use it to estimate the concentration of sugar in
sap as follows:

Equation 2: [Sap
sugar concentration] = 86/(gal sap/gal syrup)

For example, if we found that we cooked down exactly 40 gallons of sap to
produce one gallon of syrup we would know that the sugar concentration in the
sap was approximately 2.15% ([sap sugar] = 86/40).

Maple sap has little maple flavor. The distinctive flavor of the syrup is
caused by the heating which changes certain nitrogenous chemicals in the sap
(Kramer, 1983). Part of the flavor of maple syrup is due to vanillin and furanones; the darker the syrup the more the furanones and the stronger the
taste.

During the boiling process, minerals and other insoluble materials form a
sediment, called the sugar "sand," which must be filtered and removed from the
final syrup. The main constituent of the sugar sand is calcium malate (Willits,
1958).

Sap FlowSap flow requires cool nights (below freezing)
followed by warm days. In central Minnesota, sap typically flows best from
mid-March to mid-April although it can flow anytime the trees are dormant from
October to late April (Kramer and Kozlowski, 1960). Sap flow stops when the buds
expand and the leaves develop (Marvin, 1958). Flow will also stop if the
temperature is continuously above or below freezing or if the night temperatures
are no longer below freezing (Kramer and Kozlowski, 1960). At night there is
little sap flow. As the day warms, sap flow begins. By noon, approximately 60%
of the flow has occurred and the flow begins to decline (Kramer, 1983). The
temperature of the previous night appears to be one of the most important
factors for flow (Marvin, 1958).

Physiological Explanation for Sap FlowFirst, let's address a common misconception about
sap flow. Since grade school we've learned that the xylem transports water from
the roots to the aerial parts of the plant while the phloem transports sugars
and other organic materials. Though true, this has lead to the erroneous idea
that sucrose-rich maple sap is being removed from the phloem - which is wrong.
Maple sap that drips out of a spile in the tree comes from the xylem. In fact,
this is the only time during the year when the fluid in the xylem is rich in
sucrose and is an exception to the wisdom we garnered in grade school.

The cause of maple sap flow is complex and our understanding of the process is
relatively recent. Sap flow is not related to the normal process
(Cohesion-Tension
Theory) by which water is
transported in stems during the growing season (Kozlowski & Pallardy, 1997).
According to the cohesion-tension model, water is essentially "pulled" up
through a plant as it evaporates (transpiration) from leaf surfaces. Clearly
this can't be important to maple sap flow since: (1) maple trees lack leaves
during the time period when sap flows; and (2) the xylem in trees that are
transpiring and transporting water is under a negative pressure (or tension),
not a positive pressure as is measured in maple stems during sap flow.

Sap flow is not related to root pressure. Plants can generate sizable
root pressures that can play a role in water movement. In some species, like
birch (Betula sp.) and grape (Vitis sp.), the sap that flows from
cuts or wounds in the stem in the spring is a consequence of root pressure. The
root pressure increases the stem pressure which results in sap flow. However,
root pressure is not responsible for maple sap flow (Marvin, 1958;
Kramer, 1983; Kozlowski & Pallardy, 1997). Root pressure is absent in maple
trees, even when there is stem pressure (Kozlowski & Pallardy, 1997).

So, if root pressure and normal water transport mechanisms are not involved,
what causes sap flow? The crucial factor is apparently related to the age-old
observation that sap flow requires warm days and cool nights. Stems must
experience a freeze-thaw cycle for sap flow. When pieces of maple stems are
given a source of water and then placed in a freeze-thaw cycle, they exhibit sap
flow. During the cold period the stem pressure decreases and the stem absorbs
water (Kozlowski & Pallardy, 1997).

As the temperature cools, gases in the xylem dissolve and the pressure
decreases. This draws water from adjacent cells which, in turn, are refilled by
water absorbed from adjacent cells and ultimately from the root. As the
temperature continues to drop, water freezes along the inside walls of hollow
xylem cells and in the intercellular spaces. Additional ice forms as water
vaporizes from surrounding cells, much like the formation of frost on a misty
winter morning. When ice formation is complete, the remaining gases in the stem
are compressed and locked in ice. As the temperature warms, the ice melts and
the ice-compressed gases expand forcing the sap out of the stem (Tyree, 2001).

This hypothesis explains why freezing and thawing temperatures are required and
why sap flow is always followed by re-absorption of water (Marvin, 1958).
However, it doesn't explain why sap flow requires: (1) sucrose in the sap, and
(2) living cells. It is possible that both are necessary for cellular
respiration that yields carbon dioxide. This gas may be the main component of
the gases that undergo thermal expansion and contraction during the freeze-thaw
cycle (Marvin, 1958; Kramer, 1983).

The sugars in the sap are derived from carbohydrates that accumulated in the
stem during the previous season (Kramer and Kozlowski, 1960). These are
converted to starch when the weather becomes cool in the autumn. The starch in
living ray cells is hydrolyzed to sucrose as the temperature warms in the
spring. The sugary sap is then pushed into the xylem (Milburn, 1979).

Why maple?Spring sap production is a relatively rare
phenomenon, and occurs in the maples (genus Acer) and just a few others.
So, what it is about maple? According to Dr. Mel Tyree (2001) the distribution
of sap and gas in maple stems is the critical factor. Species like sugar maple
and butternut (Juglans cinerea) that have air-filled fiber cells and
water-filled vessels will exude sap. In contrast, species that do not exude sap,
such as willow (Salix), aspen (Populus), elm (Ulmus), ash (Fraxinus)
and oak (Quercus), have gas-filled vessels and water-filled fibers.

Syrup/Sap From Other SpeciesAs mentioned above, when grapes or birches are pruned in the late
spring they will exude sap. This process is not temperature dependent as is the
production of sap from maple trees and is due to root pressure. Because of the
amount of bleeding that can occur you should avoid pruning grape vines in the
late spring. Syrup can be made from birches, and is a commercially important
product in some areas.

Hickory syrup is a sugary syrup flavored by an extract of the bark of Shagbark
hickory (Carya ovata). The bark is gathered, extracted, strained and
aged.

Climate ChangeEvidence strongly suggests that global climate is changing. Climate
models suggest that winters in the Great Lakes region will likely be warmer and
wetter. One advantage of this warming trend is that it could extend the region
of maple syrup production further west in the state. Currently Minnesota is on
the western edge of the range for sugar maple trees.

Lab Activity
During today's lab we will visit and take a tour of the St.
John's Maple Syrup operation. While there, you will learn how
to identify maple trees in winter condition, tap trees, measure
sugar concentration in maple sap, and learn how to prepare maple
syrup. If time, as a group we will try to answer one selected
research question. Possible questions include:

How does sap flow relate to temperature?

What sorts of microbial
contaminants grow in the sap?

What sorts of insects are
attracted to the sap?

Do different trees vary
in sap flow?

Do trees vary in the duration of sap flow?

Do trees vary in the temperatures
at which flow occurs?

Does the sap contain cells?

Which species produces sap
with the highest sugar concentration?

Does sap flow rate vary with
species?

Is the sugar concentration of the sap higher
in large or small trees?

Is the sugar concentration greater
in the trunk or twigs?

Does tree height affect sugar
concentration or sap flow?

Is the sugar concentration in
the sap or rate of sap flow uniform along the height of the
tree?